1. Field of the Invention
This invention relates to a method and device (transmitter and receiver) for transmitting and receiving teletext data. The invention also relates to a teletext signal and to a storage medium on which such a signal is stored.
2. Description of Related Art
A known method of transmitting teletext data is described in "World System Teletext", Technical Specification, revised March 1985, herein referred to as WST specification. Section 12 of this specification describes a data format of packets X/27. These are teletext packets having magazine number X and row number 27. Such packets having a row number greater than 24 are also referred to as "ghost rows " because they are ignored by the first-generation ("Level 1") teletext decoders.
Packets X/27 provide a mechanism for accommodating references ("links") to given teletext pages in a teletext data signal. As described in the WST specification, a teletext packet X/27 comprises a maximum number of 6 references in the form of a teletext page number and sub-code. Moreover, the packet comprises a 4-bit designation code with which a maximum number of 16 different packets X/27 can be distinguished. If the designation code has one of the values 0-3, then the references are referred to as "navigational links". With these references, the editor of a teletext page can indicate which other pages the user will most likely choose as the next pages. A multipage teletext decoder may acquire the relevant pages already at an early stage so that the waiting time for linked pages can be reduced considerably. If the designation code has one of the values 4-7, then the references are referred to as "editorial links" or "compositional links". The linked pages then comprise data for enhancing the basic page. The function of packets X/27 with designation codes 8-15 has not been laid down yet. With the packets X/27 hitherto laid down, 8*6=48 different linked pages may thus be referred to within a teletext magazine.